Microlens-based retroreflective sheetings are well-known in the art. One form of such sheeting is embedded-lens sheeting wherein transparent microspheres, arranged in substantially a monolayer, are at least partially embedded in a transparent binder or spacing layer with a specularly reflecting layer behind the microspheres underlying the spacing layer. Examples of embedded-lens sheetings are disclosed in U.S. Pat. No. 2,407,680 (Palmqulst et al.) and in U.S. Pat. No. 4,511,210 (Tung et al.). Typically one or more transparent cover layers are disposed in front of the microspheres. If the microspheres are only partially embedded in the binder layer, the front of the microspheres are embedded in the cover layer. Such cover layers complete the optical relationships needed for retroreflection and may further serve to provide a protective front surface to the sheeting, such as is provided by a tough, weather-resistant, scratch-resistant polymeric cover film. Alternatively, instead of a cover film, a layer of substantially transparent adhesive may be provided in front of the microspheres, such adhesive being used, for example, to secure the sheeting to the inside of an automobile window such that the sheeting is viewed from outside the automobile.
An advantage of embedded-lens sheetings is that, because the microspheres are embedded within the sheeting, incident light rays are focused onto the specularly reflecting layer irrespective of whether the front of the sheeting is wet or dry. Thus, such sheetings are used in many applications, for instance, providing improved visibility to traffic signs, automobile license plates, etc. Such sheetings are often sold having an adhesive layer covered by a removable protective liner on the rear side of the specularly reflective layer. In use the liner is removed and the sheeting adhered to a desired substrate with the adhesive.
In order to achieve high brightness or efficient retroreflection, it is sometimes preferred to construct retroreflective sheeting with a very high density of microspheres. It is also often necessary to use dimensionally stable and what are typically very brittle and very hard materials in the spacing layer so that the dimensional relationships between the microspheres, spacing layer, and specularly reflecting layer remain constant during the conditions encountered in fabrication of a finished product with the sheeting, e.g., a license plate, and over an extended period of time despite outdoor exposure. Also, such forms of retroreflective sheeting may be thin and thereby flimsy. For all these reasons, the sheetings typically require extraordinary care in handling, and may not emboss well, suffering cracks along the outline of embossing, e.g., alphanumeric characters or other indicia. Such cracks are aesthetically undesirable and weaken the sheeting thereby rendering it more susceptible to environmental degradation.
U.S. Pat. No. 3,176,584 (DeVries et al.) discloses that a reinforcing layer may be incorporated into sheetings as taught in that patent. The reinforcing layer may be of similar composition as the binder or space coat material in which the microspheres are embedded and may be applied to the back side of the specularly reflecting layer via spraying, i.e., by a solvent-coating technique. Examples disclosed therein include methyl methacrylate, flexible epoxy resins, chloro-sulfonated polyethylene, polystyrene, polypropylene, polycarbonate resin, ethyl cellulose, and cellulose acetate-butyrate. The reference does not discuss the advantages or usefulness of such a layer, except to point out that when applied to the contoured reflecting layer, the reinforcing layer provides a flat surface. The specularly reflecting layers of retroreflective sheetings are typically very thin, i.e., on the order of 0.06 micron thick, and must be disposed in special relationship to the microspheres in order for the sheeting to provide useful retroreflection. Because the specularly reflecting layers are typically so thin, they are themselves very fragile and do not provide substantial protection to the spacing layer. Thus the sheeting may be subject to impairment of retroreflective response caused by disturbance of the specularly reflective layer and spacing layer as the reinforcing layer is applied. Such. disturbance may be particularly critical where the reinforcing layer is applied with high solvent content or at high temperature that may cause the spacing layer to deform.
U.S. Pat. No. 3,936,567 (Vesely) discloses an embedded-lens retroreflective sheeting wherein a member, described therein as an inner supporting layer, is adhered to the rear side of the specularly reflecting layer with an intermediate layer of adhesive. The inner support layers are taught by the reference to comprise a layer of plastic film such as polyvinyl chloride film, polyethylene terephthalate film, polyethylene film, nylon film, polypropylene film and similar such materials, and to generally be on the order of 5 to 15 mils thick. Adhesion of the supporting layer to the specularly reflecting layer with an intermediate layer of adhesive, as taught by the reference, gives rise to such disadvantages as increased costs for the adhesive and its application and imparts increased bulk to the sheeting, rendering it thicker and therefore more difficult to roll.
U.S. Pat. No. 3,877,786 (Booras et al.) discloses a retroreflective sheeting with a backing or reinforcing member which is intended to support and stiffen the entire article. Examples of backing materials disclosed therein include corrugated paper board, a plastic laminate reinforced with glass fibers, and metal plate. Increased stiffness of the reflective film is apparently desired to enable easier registration of the sheeting for application of one or more colored overcoats to the reflective surface thereof. Such increased stiffness, however, may be undesirable in many applications, e.g., wherein the retroreflective sheeting is to be embossed or adhered to substrate which is not flat, such as a traffic cone. Further, although the reference does not expressly so disclose, the reinforcing members disclosed therein would presumably have to be adhered to the specularly reflecting layer with an intermediate layer of adhesive.